Vincenzo Esposito. Università di Roma Tor Vergata

Transcription

1 Vincenzo Esposito Università di Roma Tor Vergata

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3 What is a fuel cell? It is an electrochemical device with a high energetic conversion yield. It convert indirectly the chemical energy of a fuel into electric power by the electrochemical reaction of the fuel with the comburent (oxidant: O 2 ). The fuel cell produces electrical power as long as the reactants are supplied to the device. Fuel Direct reaction: combustion Comburent Chemical products and heat Fuel Indirect reaction FC Electrical power Comburent

4 The Grove s cell O 2 H 2 Sir William Robert Grove ( ) H 2 SO 4 On The Correlation of Physical Forces in which he anticipated the general theory of the conservation of energy that was more famously put forward in Hermann von

5 uel Cell (proton exchange)

6 Theorical basis The fuel cells working principles are not related to the Carnot s cycle and the idea efficiency is only dependent by the temperature and by the chemical species. Fo the fuel cells, the maximum work is from a certain fuel Wel is the difference of fre energy G related to the fuel reaction with the comburent (first and second laws of th thermodynamics): f we use H 2 as fuel and O 2 as comburent: he reaction can be separated in two steps:

7 Reaction steps Fuel oxidation (H 2 or CO) at the anode lectrical current at e external circuit Ionic current at the electrolyte Comburent reduction at the cathode

8 The ionic current at the electrolyte change direction depending on the chemistry of the electrolyte materials. There are some materials which are H + conductors, OH -, CO 3 2-, other oxygen (vacancies) conductors. If H 2 is the fuel the only product is water. Water is formed at the cathode if protons are charge carrier at the electrolyte: Conversely if O 2- are c.c. at the electrolyte water is formed at the anode side:

9 Fuel cells performances Performances depend on: 1. Thermodinamics (working temperature, pressure, chemical species concentration) 2. Kinetics of the electrochemical steps

10 Thermodinamics

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17 Fuel cells thermodinamics

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22 Fuel cells Kinetics

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28 Fuel cells stack

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31 Hydrogen electrolysis

32 ydrogen production from water lectrolysis In the water at the negatively charged cathode, a reduction reaction takes place, with electrons (e ) from the cathode being given to hydrogen cations to form hydrogen gas (the half reaction balanced with acid): Cathode (reduction): 2H + (aq) + 2e H 2 (g) At the positively charged anode, an oxidation reaction occurs, generating oxygen gas and giving electrons to the cathode to complete the circuit: Anode (oxidation): 2H 2 O(l) O 2 (g) + 4H + (aq) + 4e Combining either half reaction pair yields the same overall decomposition of water into oxygen and hydrogen: Overall reaction: 2H 2 O(l) 2H 2 (g) + O 2 (g) The number of hydrogen molecules produced is thus twice the number of oxygen molecules. Assuming equal temperature and pressure for both gases, the produced hydrogen gas has therefore twice the volume of the produced oxygen gas. The number of electrons pushed through the water is twice the number of generated hydrogen molecules and four times the number of generated oxygen molecules.

33 ydrogen production from water lectrolysis

34 Fuel cell classification by the operative temperature

35 Fuel cells classification by the electrolyte materials

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41 PEMFC

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47 PEMFCs classification

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52 Direct Methanol cells Mobile Cell. 100 mw power, volume: 22 mm x 56 mm x 5mm. 2 cc fuel, lifetime 20 hours? Toshiba. Laptop Cell. Volume ~ 1 litre, powers one laptop. 10 hours fuel supply. Toshiba.

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55 Methanol Reaction schematic CH 3 OH CH 2 OH CHOH COH CH 2 OOH CHOOH COOH CH C CO 2 + H 2 O Multi-step process Several toxic organics Complex hence sluggish reaction compared to hydrogen

56 Methanol cells challenges Avoid toxicity! Control flammable vapour Methanol infrastructure toxic vapour from air breathing cells! scrub output lines with more catalysts? highly rugged technology non rechargeable cells! Avoid cell degradation carbon soot from MeOH is likely to snarl up the cell Applications Remote long-term power supply? e.g. Alaskan weather stations C. Chamberlin 2004 (Schatz Energy Research Centre):

57 SOFCs

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62 Application: mobile

63 Applicazion: stationary

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65 Design

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67 lanar

68 onolithic

69 ubular

70 egmental

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72 Single Chamber

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74 ingle chamber

75 Single chamber solid oxide fuel cell - Hibino et al. Science (2000) Fuel & oxidant mixed - no sealing issues, no coking problems Reforming done directly on anode Highly selective anode & cathode catalysts essential since fuel & oxidant exposed to both anode & cathode H 2 O + CO 2 C x H y + O 2 O 2 anode cathode electrolyte O = e - e - CH O 2 CO + 2H O 2 + 2e - O = H 2 + O = H 2 O + 2e - CO + O = CO 2 + 2e -

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78 Micro Fuel Cell

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80 icro Fuel Cell

81 icro Fuel Cell

82 icro Fuel Cell

83 icro single chamber Potential complete micropower system Polymer 3D Swiss roll Hydrocarbon fuel Single-chamber solid oxide fuel cell for power generation - direct utilization of hydrocarbons Thermal transpiration pumping of fuel/air mixture - no moving parts, uses thermal energy, not electrical energy